Qmail, as opposed to other software, was sane enough to understand that lack of MX in ANY didn't mean it wasn't there. It meant it wasn't in a cache, so it would retry with just MX.
Our initial proposal (to do REFUSED for ANY) indeed had a chance to break some older qmail installations. This is why we engaged in a longer process and found acceptable solution - HINFO. HINFO is both backwards compatible (qmail will work fine) and solves our problems with ANY. Win-win.
> Once upon a time, back in 1996, there was a really unfortunate bug in the most popular DNS server software (BIND 4.9.3): it did not respond correctly to "CNAME" requests (that is to say, requests for any CNAME data about a particular domain name). This is critical information that an email server needs to know to do its job. Thankfully, there was a way to work around the problem: "ANY" requests. These requests ask the DNS server, essentially, for any and ALL information it has about the domain name in question, including CNAME information.
> These ANY queries have two big problems:
> As you might imagine, for big domains with lots of mirrors (e.g. gmail.com), that's a lot of information, and so the response can be quite big. Big responses pose two problems: first, it's a waste of bandwidth, and second, it can expose a bug in qmail's handling of large DNS responses (see the next patch).
> ANY queries are often not cached by relaying DNS proxies (for whatever reason), and so ANY queries cause more traffic EVEN behind a caching DNS proxy server.